The term "Delayed milestone" (or "developmental delays") is used to describe the condition where a child does not reach one of the developmental milestones at the expected age. However, in most cases, a wide variety of ages can be considered normal, and not a cause for psychological or medical concern.

Milestones are often measured using percentiles, and for many milestones a value between the 5th and 95th percentile does not require intervention, though values towards the edges of that range can be associated with other medical conditions.